Interferometric alignment system for use in vacuum-based lithographic apparatus

ABSTRACT

In a lithographic apparatus having a movable object table in vacuum, an interferometer-based alignment system for detecting the position of that object table has a passive part in vacuum and an active part outside the vacuum chamber. The active part contains the beam generator, e.g. a laser, and the electronic detectors whilst the passive part contains the illumination and imaging optics. The two parts are coupled by optical fibers. The interferometer may make use of different diffraction orders from measurement and reference gratings and the order separation may be included in the passive part.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to interferometer-based alignment andposition measurement systems. More particularly, the invention relatesto such systems used in lithographic projection apparatus comprising:

an illumination system for supplying a projection beam of radiation;

patterning means, for patterning the projection beam according to adesired pattern;

a substrate table for holding a substrate; and

a projection system for imaging the patterned beam onto a target portionof the substrate.

2. Description of the Related Art

The term “patterning means” should be broadly interpreted as referringto means that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate; the term “light valve” hasalso been used in this context. Generally, the said pattern willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit or other device (seebelow). Examples of such patterning means include:

A mask table for holding a mask. The concept of a mask is well known inlithography, and its includes mask types such as binary, alternatingphase-shift, and attenuated phase-shift, as well as various hybrid masktypes. Placement of such a mask in the radiation beam causes selectivetransmission (in the case of a transmissive mask) or reflection (in thecase of a reflective mask) of the radiation impinging on the mask,according to the pattern on the mask. The mask table ensures that themask can be held at a desired position in the incoming radiation beam,and that it can be moved relative to the beam if so desired.

A programmable mirror array. An example of such a device is amatrix-addressable surface having a viscoelastic control layer and areflective surface. The basic principle behind such an apparatus is that(for example) addressed areas of the reflective surface reflect incidentlight as diffracted light, whereas unaddressed areas reflect incidentlight as undiffracted light. Using an appropriate filter, the saidundiffracted light can be filtered out of the reflected beam, leaking onthe diffracted light behind; in this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.The required matrix addressing can be performed using suitableelectronic means. More information on such mirror arrays can be gleaned,for example, from U.S. Pat. Nos. 5,296,891 and U.S. Pat. No. 5,523,193,which are incorporated herein by reference.

A programmable LCD array. An example of such a construction is given inU.S. Pat. No. 5,229,872, which is incorporated herein by reference. Forpurposes of simplicity, the rest of this text may, at certain locations,specifically direct itself to examples involving a mask table and mask;however, the general principles discussed in such instances should beseen in the broader context of the patterning means as hereabove setforth.

Also for the sake of simplicity, the projection system may hereinafterbe referred to as the “lens”; however, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The illumination system may also includecomponents operating according to any of these design types fordirecting, shaping or controlling the projection beam of radiation, andsuch components may also be referred to below, collectively orsingularly, as a “lens”. Further, the lithographic apparatus may be of atype having two or more substrate tables (and/or two or more masktables). In such “multiple stage” devices the additional tables may beused in parallel, or preparatory steps may be carried out on one or moretables while one or more other tables are being used for exposures. Twinstage lithographic apparatus are described, for example, in U.S. Pat.No. 5,969,441 and U.S. Ser. No. 09/180,011, filed Feb. 27, 1998 (WO98/40791), incorporated herein by reference.

Lithographic projection apparatus can be used, for example, in themanufacture of integrated circuits (ICs). In such a case, the patterningmeans may generate a circuit pattern corresponding to an individuallayer of the IC, and this pattern can be imaged onto a target portion(comprising one or more dies) on a substrate (silicon wafer) that hasbeen coated with a layer of radiation-sensitive material (resist). Ingeneral, a single wafer will contain a whole network of adjacent targetportions that are successively irradiated via the projection system, oneat a time. In current apparatus, employing patterning by a mask on amask table, a distinction can be made between two different types ofmachine. In one type of lithographic projection apparatus, each targetportion is irradiated by exposing the entire mask pattern onto thetarget portion at once; such an apparatus is commonly referred to as awafer stepper. In an alternative apparatus—commonly referred to as astop-and-scan apparatus—each target portion is irradiated byprogressively scanning the mask pattern under the projection beam in agiven reference direction (the “scanning” direction) while synchronouslyscanning the substrate table parallel or anti-parallel to thisdirection; since, in general, the projection system will have amagnification factor M (generally<1), the speed V at which the substratetable is scanned will be a factor M times that at which the mask tableis scanned. More information with regard to lithographic devices as heredescribed can be gleaned, for example, from U.S. Pat. No. 6,046,792,incorporated herein by reference.

There is a continuing desire in the semiconductor industry to be able tomanufacture integrated circuits (ICs) with ever higher componentdensities and hence smaller feature size. To image smaller features in alithographic projection apparatus it is necessary to use projectionradiation of shorter wavelength. A number of different type ofprojection radiation have been proposed, including Extreme Ultraviolet(EUV) in the 10-20 nm range, electron beams, ion beams and other chargedparticle fluxes. These types of radiation beam share the requirementthat the beam path, including the mask, substrate and opticalcomponents, be kept in a high vacuum. This is to prevent absorptionand/or scattering of the beam and a total pressure of less than about10⁻⁶ millibar is necessary. Optical elements for EUV radiation can bespoiled by the deposition of carbon layers on their surface whichimposes the additional requirement that hydrocarbon partial pressuresmust be kept below 10⁻⁸ or 10⁻⁹ millibar.

Working in such a high vacuum imposes quite onerous conditions on thecomponents that must be put into the vacuum and on the vacuum chamberseals, especially those around any part of the apparatus where a motionmust be fed-through to components inside the chamber from the exterior.For components inside the chamber, materials that minimize or eliminatecontaminant outgassing, either from the materials themselves or fromgases adsorbed on their surfaces, must be used.

It also is well known that the substrate (wafer) that is being exposedmust be positioned to extremely high accuracy relative to the mask(reticle). A wafer may undergo 20 or 30 exposures during the manufactureand it is essential that the various images are properly aligned, evenif different lithography apparatus are used for different exposures. Theoverlay accuracy requirements only increase with reduced feature sizeand shorter wavelength radiation.

An object of the present invention is to provide an alignment and/orposition measuring system capable of measuring the position of an objectin vacuum with high accuracy, e.g. for use in a lithographic projectionapparatus.

According to the present invention there is provided a lithographicprojection apparatus comprising:

a an illumination system for supplying a projection beam of radiation;

patterning means, for patterning the projection beam according to adesired pattern;

a substrate table for holding a substrate; and

a projection system for imaging the patterned beam onto a target portionof the substrate; characterized by:

a vacuum chamber in which at least one of said patterning means and saidsubstrate table is contained, said object table being movable; and

an alignment system constructed and arranged to align said patterningmeans and a substrate on said substrate table, said alignment systemcomprising a passive part contained in said vacuum chamber and an activepart outside said vacuum chamber.

SUMMARY OF THE INVENTION

By positioning only the passive part of the alignment system inside thevacuum chamber, the present invention avoids difficulties in making theactive part of the alignment system vacuum compatible and reduces heatand vibration generation in the vacuum system, which may disturb theexposure and cause positioning uncertainties.

In embodiments of the present invention, the active and passive parts ofthe system are coupled together by optical fibers. The alignment systemmay be an interferometer system for detecting the position of ameasurement grating (wafer mark) relative to a reference grating. Such asystem may image at least two orders of radiation diffracted by themeasurement grating onto the reference grating and may comprise beamsplitting means to separate radiation diffracted by the referencegrating and deriving from one order diffracted by the measurementgrating from diffracted radiation deriving from other orders diffractedby the measurement grating.

According to a further aspect of the invention there is provided adevice manufacturing method comprising the steps of:

providing a substrate that is at least partially covered by a layer ofradiation-sensitive material;

providing a projection beam of radiation using an illumination system;

using patterning means to endow the projection beam with a pattern inits cross-section;

projecting the patterned beam of radiation onto a target portion of thelayer of radiation-sensitive material; and

providing a vacuum chamber which comprises a movable substrate table forholding the substrate; characterized by the step of:

prior to or during said step of irradiating and imaging, aligning saidpatterning means and substrate on said substrate table using analignment system comprising a passive part provided in said vacuumchamber and an active part provided outside said vacuum chamber.

In a manufacturing process using a lithographic projection apparatusaccording to the invention a pattern in a mask is imaged onto asubstrate which is at least partially covered by a layer ofradiation-sensitive material (resist). Prior to this imaging step, thesubstrate may undergo various procedures, such as priming, resistcoating and a soft bake. After exposure, the substrate may be subjectedto other procedures, such as a post-exposure bake (PEB), development, ahard bake and measurement/inspection of the imaged features. This arrayof procedures is used as a basis to pattern an individual layer of adevice, e.g. an IC. Such a patterned layer may then undergo variousprocesses such as etching, ion-implantation (doping), metallization,oxidation, chemo-mechanical polishing, etc., all intended to finish offan individual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

Although specific reference may be made in this text to the use of theapparatus according to the invention in the manufacture of ICs, itshould be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetarea”, respectively.

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation or particle flux,including, but not limited to, ultraviolet (UV) radiation (e.g. at awavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), extremeultraviolet (EUV) radiation, X-rays, electrons and ions.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described below with reference toexemplary embodiments and the accompanying schematic drawings, in which:

FIG. 1 depicts a lithographic projection apparatus according to a firstembodiment of the invention;

FIG. 2 is a plan view of the substrate table of the first embodiment ofthe invention;

FIG. 3 is a diagram of the optical components of an alignment system inthe first embodiment of the invention;

FIG. 4 is an enlarged view of the reference mark used in the firstembodiment of the invention;

FIG. 5 is diagram of the optical components in vacuum in an alignmentsystem according to a second embodiment of the invention; and

FIG. 6 is a diagram of the optical components outside vacuum in thesecond embodiment of the invention.

In the drawings, like references indicate like parts.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiment 1

FIG. 1 schematically depicts a lithographic projection apparatusaccording to the invention. The apparatus comprises:

a radiation system LEX, LS, IN, CO for supplying a projection beam PB ofradiation (eg. UV or EUV radiation);

a first object table (mask table) MT provided with a mask holder forholding a mask MA (e.g. a reticle), and connected to first positioningmeans for accurately positioning the mask with respect to item PL;

a second object table (substrate table) WT provided with a substrateholder for holding a substrate W (e.g. a resist-coated silicon wafer),and connected to second positioning means for accurately positioning thesubstrate with respect to item PL;

a projection system (“lens”) PL (e.g. a refractive or catadioptricsystem, a mirror group or an array of field deflectors) for imaging anirradiated portion of the mask MA onto a target portion C of thesubstrate W. As here depicted, the apparatus is of a transmissible type(i.e. has a transmissive mask). However, in general, it may also be of areflective type, for example.

The radiation system comprises a source LA, (e.g, a Hg lamp, excimerlaser, an undulator provided around the path of an electron beam in astorage ring or synchrotron, a plasma source, or an electron or ion beamsource) which produces a beam of radiation. This beam is passed alongvarious optical components comprised in the illumination system,—e.gbeam shaping optics EX, an integrator IN and a condenser CO—so that theresultant beam PB has a desired shape and intensity distribution in itscross-section.

The beam PB subsequently intercepts the mask MA which is held in a maskholder on a mask table MT. Having passed through the mask MA, the beamPB passes through the lens PL, which focuses the beam PB onto a targetportion C of the substrate W. With the aid of the interferometricdisplacement measuring means IF and the second positioning means, thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, theinterferometric displacement means IF and the first positioning meanscan be used to accurately position the mask MA with respect to the pathof the beam PB, e.g. after mechanical retrieval of the mask MA from amask library. In general, movement of the object tables MT, WT will berealized with the aid of a long stroke module (course positioning) and ashort stroke module (fine positioning), which are not explicitlydepicted in FIG. 1.

The depicted apparatus can be used in two different modes:

1. In step mode, the mask table MT is kept essentially stationary, andan entire mask image is projected in one go (i.e. a single “flash”) ontoa target portion C. The substrate table WT is then shifted in the xand/or y directions so that a different target portion C can beirradiated by the beam PB;

2. In scan mode, essentially the same scenario applies, except that agiven target portion C is not exposed in a single “flash”. Instead, themask table MT is movable in a given direction (the so-called “scandirection”, e.g. the x direction) with a speed v, so that the projectionbeam PB is caused to scan over a mask image; concurrently, the substratetable WT is moved in the same or opposite direction at a speed V=Mv, inwhich M is the magnification of the lens PL, (typically, M=¼ or ⅕). Inthis manner, a relatively large target portion C can be exposed, withouthaving to compromise on resolution.

During projection of the mask image onto the target portion C, the maskMA and the substrate W have to be aligned correctly. A correct alignmentmay be achieved by aligning one or more markers M₁, M₂ provided on themask MA with respect to one or more corresponding reference markersM_(R) provided on wafer table WT, as shown in FIG. 2. The position ofthe substrate table with respect to the mask will then be known. Such analignment may be achieved by using projection (actinic) radiation andthe projection system to image the markers M₁, M₂, onto the referencemarker M_(R). The reference marker may take the form of an image sensorprovided on the substrate (wafer) table WT. Details of such an imagesensor are not shown in the figures. More information on an image sensorcan be gleaned from European Patent Application 00202960.1 incorporatedherein by reference (applicant's ref.: P-0203). The mask table and/orthe wafer table may be moved to obtain an aligned position, which willbe recorded with the aid of the interferometric displacement measuringsystem IF. However, other schemes for aligning mask MA and substratetable WT may also be employed.

Further, the position of the wafer W with respect to the wafer table WTmust be known in order to align the wafer W with respect to the mask MA.An additional alignment system 10, which embodies the present invention,is used for this purpose. With the alignment system 10, one or moremarkers P₁, P₂, P, in the form of diffraction gratings provided on thewafer and wafer table will be aligned with respect to a reference markeror grating 13 of the alignment system 10 by moving the wafer table WT.Recording the respective aligned positions with the aid of theinterferometric displacement measuring means IF will then yield aposition of the wafer with respect to the wafer table. The markers P andM_(R) on the wafer table WT are shown in FIG. 2 on a common plate, whichis mounted on the wafer table. An arrangement of this type, in whichpart or all of the alignment process can be carried out away from themain axis of the machine (the optical axis of the projection system PL),is sometimes referred to as an “off-axis” alignment system.

The wafer W and the wafer table WT are contained in a vacuum chamber VCthat is maintained at a vacuum during operation of the apparatus. Thealignment system 10 comprises a passive part 10 a enclosed in the vacuumchamber and an active part 10 b disposed outside the vacuum chamber VC.

The optical components of the alignment system 10 are shown in FIG. 3.The passive part 10 a enclosed in the vacuum chamber VC of alignmentsystem 10 comprises three main sections; illumination branch 20, imagingbranch 30 and detection branch 40. These branches respectively performthe functions of: collimating and adjusting the measurement beam 12;projecting the measurement beam 12 onto the wafer marker P and imagingthe diffracted radiation onto the fixed reference grating 13; andseparating the resultant diffraction beams so that they can be convertedinto electronic signals for use by the control system of the lithographyapparatus. The active part 10 b provided outside the vacuum chamber VCin this embodiment consists of the laser module 80 shown in FIG. 3 andphotodetectors (not shown) for converting the sub-beams separated by thedetection branch into electronic control signals.

The laser module 80 includes laser 81 which is, for example, a 10 mWHeNe laser outputting a beam of wavelength 633 nm. The beam is firstpassed through a safety shutter 82, Faraday isolator 90, modulator 83,polarizing beam splitter 91 and attenuator 84, which is provided forcontrol purposes. The safety shutter 82 completely blocks the laser beamwhen it is not required, and particularly when the apparatus has beenopened up, e.g. for maintenance, for safety reasons. Faraday isolator 90prevents unwanted back reflections from any of the optical components inthe system reaching the laser 81 and disturbing its operation, e.g. bycausing frequency pulling or mode hopping, etc. Piezo-electric modulator83 and polarizing beam splitter 91 are used in a modulation-demodulationdetection scheme to enhance the signal to noise ratio of the sensor. Thebeam is coupled into a single-mode polarization-preserving fiber 21,which is optimized for the wavelength of the radiation, via fibermanipulator 85 and is taken into the vacuum chamber. The laser module 80can also conveniently include the laser power supply 86, modulatordriver 87 and control electronics 88 in the same package.

In the illumination branch 20, situated in the vacuum chamber VC, themeasurement beam 12 exits the fiber terminator 22 that includescollimating optics to provide a collimated beam. The collimated beam isfocused at the center of the pupil plane of the imaging branch 30 by aplano-convex lens 23. A first plane-plate 24 before plano-convex lens 23is used to adjust the angle of the measurement beam at the imagingbranch pupil and hence the position of the beam at the plane of thewafer. An initial, coarse, adjustment of this can be done by X-Ytranslation of the fiber optics output. A second plane-plate 25 ispositioned after the plano-convex lens 23 and used to adjust theposition of the measurement beam 12 at the pupil plane and hence itsangle of incidence at the plane of the wafer. Finally, a 90° mirror 26brings the measurement beam into the imaging branch 30.

Having entered the imaging branch, the measurement beam is coupled alongthe optical axis (Z-axis) using a small mirror 31. Mirror 31 isconveniently mounted in the center of first order-diaphragm 32, whosepurpose is described below.

The imaging branch is a 4-f double telecentric optical system withmagnification M=−1. It contains optics in the form of first and secondair-spaced doublets 33, 34 each having a focal length of about 50 mm andbeing made of SF1. The use of air-spaced doublets is preferred sinceoptical cements, such as would be found in a simpler achromatic doublet,may not be vacuum compatible. The accuracy of the spacing of thesinglets in the doublet is a major determinant of system performance andis assured by using accurately machined ceramic spacer balls (the errorin the radius being<1 μm); this guarantees high mounting accuracy (i.e.spacing distance), vacuum compatibility and thermal stability. SF1 is aheavy flint glass having a refractive index of about 1.7 which allowsthe lenses to have a suitable focal length without an excessive radiusof curvature. The symmetry of the system reduces any aberrations causedby uncertainty in the refractive index of the SF1 glass. To this end thelenses should all be made from the same batch of glass.

First order-diaphragm 32, bearing mirror 31 is mounted between first andsecond doublets 33, 34 so that the measurement beam is collimated onwafer marker P on wafer W by first doublet 33. Front mirror 35 isprovided so that the alignment system can be positioned in a convenientlocation and the measurement beam 12 is incident normally on thereference mark P. Front mirror 35 may be a Zerodur (TM) substrate (forthermal stability) with metallic coating for efficient reflection of theS-polarized illumination beam at 45° angle of incidence as well as thereturning diffraction orders at angles of<about 54°. The mirror may bemounted on a Zerodur (TM) frame for additional thermal stability.

At the wafer marker P, the illumination beam is reflected and diffractedinto diffraction orders at specific angles in the XZ and YZ planes. Thefirst doublet 33 has an aperture sized to select the diffraction ordersup to and including the fourth and focuses the collimated orders in itsback focal plane. The parallel orders pass through first order-diaphragm32 that includes apertures which pass only the first 12 a and fourth 12b diffraction orders. A linear polarizer 36 is used to clean-up thepolarization of both orders 12 a, 12 b, since this may have becomeslightly elliptical on reflection by front mirror 35, after which onlythe first order 12 a is passed through a half-wave plate 37 so that thelinear polarization states of the two orders are 90° apart. Polarizer 36may be formed of borosilicate glass with aligned silver particles, whichis a vacuum compatible component, and effectively reduces cross-talkbetween the two beams. Half-wave plate 37 may be a quartz plate ofappropriate thickness for the single wavelength of the measurement beam12.

Second lens doublet 34 images the first and fourth orders 12 a, 12 bonto its back focal plane in which the fixed reference grating 13 ispositioned. The reference pattern of reference grating 13 is a hard copyformed of chromium on glass of the aerial image of the plus and minussub-beams of the first-order beams from the wafer marker P. During X-Ymovement of the wafer and wafer table WT, and hence wafer marker P, forexample during an alignment scan, the total image of the wafer marker atthe reference pattern will move correspondingly. The transmitted lightis then captured and measured by the detection branch 40.

At the reference grating 13, the light in the first-order beam 12 a willbe diffracted into diffraction orders forming beams which are spatiallywell-separated from the beams formed of the diffraction orders intowhich the light in the fourth-order beam 12 b is diffracted. Diffractionorders of the plus and minus sub-beams of the first-order beam 12 a willoverlap as will diffraction orders of the plus and minus sub-beams ofthe fourth order beam 12 b. As well as being spatially well-separated,the diffraction components deriving from the first-order beam 12 a andthose deriving from the fourth-order beam 12 b will have linearpolarizaton states differing by 90°, since the linear polarizationstates of the first-order beam 12 a and the fourth-order beam 12 b are90° apart. In the detection branch 40, diffraction components derivingfrom the first-order beam 12 a and those deriving from the fourth-orderbeam 12 b are separated into first and second signal sub-beams 12 c, 12d respectively by first and second polarizing beam splitters 41, 42 andfurther order-diaphragms 43, 44.

To achieve this separation, the beams diffracted by the referencepattern 13 are first collimated by a second lens 45 and then areincident on first polarizing beam splitter 41 which substantially passesthe components deriving from the first-order beam 12 a and substantiallydeflects those deriving from the fourth-order beam 12 b. Selectedcomponents deriving from the first-order beam 12 a pass through secondorder-diaphragm 43 (components deriving from the fourth-order beam 12 bbeing blocked) and are then imaged by third lens 46 on a set of fouroptical fibers or fiber bundles, comprising first detection fiber set47, mounted in fiber terminator 47 a. The positioning of these fibers,or fiber bundles, corresponds to the four quadrants of the referencemark P, described below.

The components deriving from fourth-order beam 12 b are substantiallydeflected by first polarizing beam splitter 41 to second polarizing beamsplitter 42 which diverts them through third order-diaphragm 44 whichonly passes selected components deriving from the fourth-order beam 12b. A third lens 48 images the selected components onto a seconddetection fiber set 49 mounted in fiber terminator 49 a.

Such components as pass through the second beam splitter 42 areprojected by fourth lens 50 and a 90° bending prism 51 onto a coherentfiber bundle 52 mounted in terminator 52 a. The resulting image beam,third signal sub-beam 12 e, carries an image of the reference pattern13.

The first, second and third signal sub-beams 12 c, 12 d, 12 e are takenout of the vacuum chamber by their respective fiber bundles 47, 49, 52.The first and second signal sub-beams are passed to detectors of knowntype to make an accurate measurement of the reference mark position in aknown alignment scan process. The third signal sub-beam is taken to aCCD camera to provide a visual indication of the alignment to anoperator of the apparatus.

FIG. 4 is an aerial view of the wafer marker P showing its fourquadrants Pa, Pb, Pc, Pd. These are arranged such that two diagonallyopposite quadrants Pa, Pc have grating lines parallel to the X-axiswhilst the other two quadrants Pb, Pd have grating lines parallel to theY-axis.

Embodiment 2

A second embodiment of the invention is illustrated in FIGS. 5 and 6.This embodiment shares a number of components with the first embodimentand parts not specifically described below are similar to thecorresponding parts of the first embodiment. For example, the lasermodule 80, illumination branch 20 and imaging branch 30 in the secondembodiment are essentially the same as those of the first embodiment.The major difference between the two embodiments is that the detectionbranch 70 is located outside the vacuum chamber VC.

As shown in FIG. 5, in the second embodiment, the light passing throughthe reference grating 13, which represents a combined image of gratingsP and 13, is collected by a fiber taper 60 which magnifies the image anddelivers it into fiber bundle 61 to be taken out of the vacuum chamberto detection branch 70, shown in FIG. 6. In the fiber taper the fiberends are tapered and packed closer together so that the image grows asit is transmitted down the bundle.

Fiber bundle 61 terminates at a terminator 62 and emits the combinedimage signal through window 63 in the wall of the vacuum chamber VC. Onthe other side of the vacuum wall, the combined image is projected bylenses 71, 72 onto a photodiode detector 73 of known type, including apre-amp, which provides the electronic alignment signal for theapparatus control systems. The photodiode detector 73 has four quadrantsto separately detect the four gratings of the wafer and referencegratings P, 13. To provide a visual indication of alignment to theoperator, a portion of the combined image signal is diverted by apolarizing beam splitter 74 positioned between lenses 71, 72. This beamis focused by lens 75 onto a cross-hair reference 76 at the focus oflens 77 that then focuses the beam onto CCD camera 78. For conveniencein arranging the components of detection branch 70, a corner prism 79 isincluded after lens 75 to bring the camera branch parallel to the signalbranch.

FIG. 6 also shows how the light from laser module 80 is taken via fiber89 to window 64 in vacuum chamber wall thorough which it is transmittedto fiber 21 leading to the illumination branch in the vacuum chamber.

In a variation of the second embodiment, the first-order andfourth-order beams can be separated and separately detected outside thevacuum chamber VC for improved alignment accuracy, provided that fiber61 is polarization preserving.

Whilst we have described above a specific embodiment of the invention itwill be appreciated that the invention may be practiced otherwise thandescribed. The description is not intended to limit the invention. Inparticular it will be appreciated that the invention may be used witheither or both the substrate or mask table of a lithographic apparatus.

What is claimed is:
 1. A lithographic projection apparatus comprising: a projection beam illumination system which supplies a projection beam of radiation; a projection beam patterning device which patterns the projection beam according to a desired pattern; a substrate table for holding a substrate; and a projection system which images the patterned beam onto a target portion of the substrate; a vacuum chamber in which at least one of said projection beam patterning device and said substrate table is contained; and an alignment system constructed and arranged to align said projection beam patterning device and a substrate on said substrate table, said alignment system comprising a passive part contained in said vacuum chamber and an active part outside said vacuum chamber, wherein said active part comprises a measurement beam generator constructed and arranged to generate a measurement beam of radiation and a detector; and said passive part comprises optical components constructed and arranged to direct said measurement beam onto a mark, to receive radiation reflected thereby and to direct it into a signal beam.
 2. Apparatus according to claim 1 wherein said passive part comprises essentially only passive optical components and support structures.
 3. Apparatus according to claim 2 wherein said passive optical components comprise essentially only components selected from the group comprising: reflecting, diffracting, refracting, directing, selecting, polarizing and filtering components.
 4. Apparatus according to claim 1 wherein said passive part contains essentially no light generating components.
 5. Apparatus according to claim 1 wherein said passive part contains essentially no electronic current carrying components.
 6. Apparatus according to claim 1 wherein said alignment system comprises an off-axis alignment system constructed and arranged to align the substrate on the substrate table with respect to a reference.
 7. The apparatus according to claim 1, wherein said alignment system is an optical interferometer system, and said passive part comprises: an order diaphragm constructed and arranged to select at least two different orders of radiation diffracted by said mark in the form of a measurement grating, said selected orders comprising plus and minus diffracted sub-beams; a reference grating; and an optical system constructed and arranged to direct the selected orders onto said reference grating, wherein radiation diffracted by said reference grating forms said signal beam.
 8. Apparatus according to claim 7 wherein said alignment system further comprises a beam splitter constructed and arranged to separate said signal beam into multiple sub-beams, each separated signal sub-beam comprising radiation diffracted by said reference grating and deriving from one order diffracted by said measurement grating.
 9. Apparatus according to claim 7 wherein said reference grating is substantially a hard copy of an aerial image of the plus and minus sub-beams of the lowest order diffracted by said measurement grating and selected by said order diaphragm.
 10. Apparatus according to claim 9 wherein said order diaphragm is constructed and arranged to select an even and an odd order diffracted by said measurement grating.
 11. Apparatus according to claim 10 wherein said order diaphragm is constructed and arranged to select the first and fourth orders diffracted by said measurement grating.
 12. Apparatus according to claim 10 wherein said alignment system comprises a polarization director constructed and arranged to have the two selected orders in linear polarization states that are 90° apart; and said beam splitter comprises at least one polarizing beam splitter constructed and arranged to selectively pass or deflect radiation diffracted by said reference grating and deriving from one order diffracted by said measurement grating.
 13. Apparatus according to claim 12 wherein said polarization director comprises a linear polarizer and a half-wave plate.
 14. Apparatus according to claim 10 wherein said beam splitter further comprises diaphragms arranged behind said at least one polarizing beam splitter and constructed to selectively pass radiation diffracted by said reference grating and deriving from one order diffracted by said measurement grating.
 15. Apparatus according to claim 8 wherein said beam splitter forms part of said passive part; and said alignment system further comprises a first optical fiber constructed and arranged to couple said measurement beam from said active part to said passive part and a second optical fiber [means] comprising separate optical fibers or fiber bundles, each constructed and arranged to couple one of said separated signal sub-beams from said passive part to said active part.
 16. The apparatus according to claim 1, wherein said alignment system comprises a first optical fiber constructed and arranged to couple said measurement beam from said active part to said passive part and a second optical fiber constructed and arranged to couple said signal beam from said passive part to said active part.
 17. The apparatus according to claim 1, wherein said alignment system comprises an optical fiber constructed and arranged to couple an image of said reference grating to said active part, and said active part comprises a photodetector constructed and arranged to detect said signal beam from said optical fiber.
 18. Apparatus according to claim 1 wherein said active part comprises a camera. 